1. Field of the Invention
Embodiments of the present invention are directed in general to novel aluminate-green phosphors (herein referred to as green phosphors). Specifically, embodiments of the present invention are directed to use of the novel aluminate-based green phosphors in display applications, such as for back lighting in liquid crystal displays (LCD's), plasma display panels (PDP's), and cathode ray tube (CRT) displays, but also toward isolated green LED's, white light illumination systems, signal lights, and pointers.
2. State of the Art
Embodiments of the present invention are directed to green phosphors, which provide an alternative to the green LEDs of the art. Green LEDs have the disadvantage of being notoriously less efficient than their UV, blue, and red LED counterparts, and additionally, emitted radiation from a green LED can exhibit a wavelength shift with increasing temperatures, an undesirable characteristic. A green phosphor used in conjunction with a UV to blue light emitting diode that provides the excitation radiation to the phosphor, however, provides a device that addresses many of the problems of the green LED. Such a device takes advantage of the so-called down-conversion process, where UV to blue light emitted from the LED can be converted to green light via the green phosphor. Specifically, such devices utilizing green phosphors can be capable of providing efficiencies compatible to a blue LED, where “efficiency” refers to the number of photons emitted by the phosphor relative to the number of photons initially provided as excitation energy. Of course, it will be understood by those skilled in the art that excitation of an LED is carried out with electric energy, so in this sense “efficiency” means power conversion.
Green phosphors have been previously described in the art. During the early phases of the development of these compositions, it was known that a luminescent material could be generated with a base material into which a suitable activator was incorporated. The base materials were aluminate oxides or silicates of alkaline earth metals, and the activator was the rare earth element europium in a +2 valence state (e.g., Eu2+). An early disclosure by H. Lange in U.S. Pat. No. 3,294,699, for example, described a strontium aluminate composition activated with an europium (II) oxide, wherein the amount of the europium oxide added to the strontium aluminate was between about 2 and 8 mol percent. A specific luminescent material was 0.9 SrO.Al2O3.0.03 EuO, which was shown to emit light in a broad band spectrum having a peak response in the green region of about 520 millimicrons when excited by the mecury line at 365 millimicrons (nanometers).
Subsequent to this disclosure a number of different europium activated aluminate compositions appeared in the literature. These compositions were described in relation to a number of different end-use applications, but considering the so-called quenching effect that the europium in relation to luminescent properties, the amount of the europium that appears in the phosphor compositions has been maintained in the past at relatively low levels.
An illumination system for use as the green light of a traffic light or in automotive display applications has been described in U.S. Pat. No. 6,555,958 to A. M. Srivastava et al. Disclosed in this patent were both silicate and aluminate-based blue-green phosphors, the aluminate-based compositions being generally represented by the formula AAlO:Eu:2+, where A comprised at least one of Ba, Sr, or Ca. The preferable composition disclosed in this patent was AAl2O4:Eu2+, where A comprised at least 50% Ba, preferably at least 80% Ba and 20% or less Sr. When A comprised Ba, the phosphor peak emission wavelength was about 505 nm and the phosphor quantum efficiency was “high.” When A comprised Sr, the phosphor peak emission wavelength was about 520 and the phosphor quantum efficiency was “fairly high.” Thus, it was disclosed by this patent that A most preferably comprised Ba to obtain a peak wavelength closest to 505 nm and to obtain the highest relative quantum efficiency. Further revealed was that in the alkaline earth aluminate phosphor, the europium activator substitutes on the alkaline earth lattice site, such that the phosphor may be written as (A1−xEux)Al2O4, where 0<x<0.2. The most preferred phosphor composition was (Ba1−xEux)Al2O4, where 0<x<0.2. The compositions disclosed in this patent did not contain magnesium or manganese.
An alkaline earth aluminate compound in which the alkaline earth was a magnesium-containing compound containing no fluorine atoms in its molecules was disclosed in U.S. Pat. No. 5,879,586 to K. Kitamura et al. The rare earth component of this phosphor was cerium and terbium, according to the formula (Ce1−wTbw)MgxAlyOz, where 0.03≦w≦0.6; 0.8≦x≦1.2; 9≦y≦13; and 15≦z≦23. This terbium containing compound was reported to emit “high-luminance green light,” but relative intensities and peak emission wavelengths were not given, and this green light emitting compound did not contain europium as an activating rare earth element. A “high-luminescence blue-green” emitting phosphor based on strontium as the alkaline earth and europium as the activator was expressed by the formula (Sr4(1−w)Eu4w)AlxOy, where 0.01<w<0.6; 11≦x≦17; and 20≦y≦30 but again, relative intensities and peak emission wavelengths were not given.
Green phosphors based on thiogallates have been disclosed. In U.S. Pat. No. 6,686,691 to G. O. Mueller et al., a device comprising a green phosphor and a blue LED (the green phosphor absorbing blue light from the blue LED) was disclosed. In one embodiment, the green phosphor was based on a host sulfide material; in other words, a lattice which included sulfide ions. A preferred host sulfide material was a thiogallate such as SrGa2S4, and when activated by the rare earth europium, the green phosphor SrGa2S4:Eu demonstrated a spectrum having a luminous equivalent value of about 575 lm/W at a maximum wavelength of about 535 nm. The dopant (rare earth Eu) concentration in the SrGa2S4 host was preferably from about 2 to 4 mol %. The blue LED providing the excitation radiation to the green phosphor was an (In,Ga)N diode emitting radiation at a wavelength from about 450 to 480 nm.
A similar strontium thiogallate based phosphor used as a backlight for an LCD has been described by C. H. Lowery in published U.S. application 2004/0061810. In this disclosure, the wavelength-converting material selected to absorb light emitted by the active region of the LED die could be either the strontium thiogallate phosphor described above, or a nitridosilicate phosphor. The strontium thiogallate phosphor had a dominant emission wavelength of about 542 nm. The wavelength-converting material absorbed blue light from the LED die either in a region from about 420 to 460 nm, or, in other embodiments, in a region ranging from about 380 to 420 nm. Again, these devices comprising green-emitting phosphors eliminated problems encountered with green LEDs, such as high temperature stability, and temperature-induced variations in color.
U.S. Pat. No. 6,805,814 to T. Ezuhara et al. describe a green light emitting phosphor or use in plasma displays, the phosphor represented by the formula M11−aM211−bMna+bO18−(a+b)/2, where M1 is at least one of La, Y, and Gd, and M2 is at least one of Al and Ga. In cases where the phosphor contains Al (e.g., wherein the phosphor is an aluminate), the alumina has a purity of not less than 99.9%, and a crystal structure of either a alumina or an intermediate alumina such as aluminum hydroxide. The peak emission wavelengths of these green light emitting phosphors was not given. The excitation wavelengths were in the vacuum ultraviolet.
The green phosphors of the prior art suffer from two drawbacks: 1) many emit in a wide band spectrum, which, while generally desirable for achieving a higher color rendering in white light illumination sources, is not appropriate for liquid crystal display (LCD) backlighting, plasma display panels (PDPs) and cathode ray tubes (CRTs), and 2) the luminescent intensity (e.g., brightness) and conversion efficiency of the prior art green phosphors is less than adequate. A “wide band spectrum” for the present purposes may be described as a peak in a spectrum demonstrating a full width at half maximum (FWHM) greater than about 80 nm. For display applications, the color space is dependent on the positions of the individual red (R), green (G), and blue(B) components as represented by their color coordinates. To achieve a larger color space, or “wide color gamut display,” as it is known in the art, it is desirable to provide a green phosphor that emits at a peak wavelength of about 520 nm, the peak having a FWHM preferably less than about 80 nm, and with the bandwidth of the spectrum covering at least some of the turquoise color, without sacrificing illumination intensity.
Therefore, what is needed in the art is a green phosphor having color coordinates around the values of x=0.193 and y=0.726, a peak emission wavelength around 518 nm wherein the phosphor emits in a narrow wavelength range, and an emission intensity greater than that provided by any known green phosphor in the art. In combining the present green phosphors with a high efficiency UV and blue GaN based LED, a color stable and highly efficient green LED may be provided.